# Nexus Go Quickstart

> Build a Nexus Service that wraps an existing Temporal Workflow using the Go SDK

[Temporal Nexus](/evaluate/nexus) connects Temporal Applications within and across Namespaces using a Nexus Endpoint, a Nexus Service contract, and Nexus Operations. Build a Nexus Service that wraps an existing Temporal Workflow, then invoke it from a caller Workflow.

> **ℹ️ Info:**
> NEW TO NEXUS?
>
> This page will help you get a working sample running in Go. 
> To evaluate whether Nexus fits your use case, see the [evaluation guide](/evaluate/nexus) and to learn more about Nexus features, click [here](/nexus).
>

**Prerequisites:** Complete the [Go SDK Quickstart](/develop/go/set-up-your-local-go) first.
You should have `activity.go`, `workflow.go`, `worker/main.go`, and `start/main.go` from that guide.

## What you'll build

You have `SayHelloWorkflow` running in the `default` Namespace.
By the end of this guide:

1. A Nexus Service will expose `SayHelloWorkflow` as an Operation.
2. A second Namespace will contain a Workflow that calls that Operation.
3. The caller Workflow will get back `"Hello Temporal"` — the same result, but across Namespaces.

## 1. Define the Nexus Service

Create a file called `service.go` that defines the Nexus Service contract.

Creating a Nexus Service establishes the contract between your implementation and any callers.
It provides type safety when invoking Nexus Operations and ensures that Operation Handlers fulfill the contract.

`HelloServiceName` and `HelloOperationName` are string constants that uniquely identify the Service and Operation.
`HelloInput` defines the input type for the Operation.
`SayHelloWorkflow` returns `string`, so the Operation output type is also `string`.

```go
{`package greeting

const (
    HelloServiceName   = "my-hello-service"
    HelloOperationName = "say-hello"
)

type HelloInput struct {
    Name string
}`}
```

## 2. Define the Nexus Operation handlers

Create a file called `nexus_handler.go` that implements the Nexus Operation handler.

Operation handlers contain the logic that runs when a caller invokes a Nexus Operation.

`HelloNexusWorkflow` acts as the handler Workflow.
It bridges the Nexus `HelloInput` to `SayHelloWorkflow`'s `string` parameter by extracting `input.Name`.

`temporalnexus.NewWorkflowRunOperation` creates an asynchronous Nexus Operation that starts `HelloNexusWorkflow` when invoked.
The Options function returns `client.StartWorkflowOptions`, including a stable Workflow ID derived from `options.RequestID`.

```go
{`package greeting

import (
    "context"

    "github.com/nexus-rpc/sdk-go/nexus"
    "go.temporal.io/sdk/client"
    "go.temporal.io/sdk/temporalnexus"
    "go.temporal.io/sdk/workflow"
)

// HelloNexusWorkflow is the handler Workflow for the Nexus SayHello Operation.
// It bridges the Nexus HelloInput to SayHelloWorkflow's string parameter.
func HelloNexusWorkflow(ctx workflow.Context, input HelloInput) (string, error) {
    return SayHelloWorkflow(ctx, input.Name)
}

var HelloOperation = temporalnexus.NewWorkflowRunOperation(
    HelloOperationName,
    HelloNexusWorkflow,
    func(ctx context.Context, input HelloInput, options nexus.StartOperationOptions) (client.StartWorkflowOptions, error) {
        return client.StartWorkflowOptions{
            // RequestID is stable across retries, making it safe to use as a Workflow ID.
            ID: options.RequestID,
        }, nil
    },
)`}
```

## 3. Register the Nexus Service handler in a Worker

Update your existing `worker/main.go` to register the Nexus Service Handler.

This Worker runs in the `default` Namespace — the same Namespace where `SayHelloWorkflow` is already registered.

A Worker will only poll for and process incoming Nexus requests if the Nexus Service Handlers are registered.
This is the same Worker concept used for Workflows and Activities.

`nexus.NewService` creates a named Nexus Service.
`service.Register` adds the `HelloOperation` to the Service.
`w.RegisterNexusService` registers the Service with the Worker so it can receive Nexus Operation requests.
`HelloNexusWorkflow` must also be registered so the Worker can execute it as the handler Workflow.

```go
{`package main

import (
    "log"

    "my-org/greeting"

    "github.com/nexus-rpc/sdk-go/nexus"
    "go.temporal.io/sdk/client"
    "go.temporal.io/sdk/worker"
)

func main() {
    // Empty Options defaults to the "default" Namespace.
    c, err := client.Dial(client.Options{})
    if err != nil {
        log.Fatalln("Unable to create client", err)
    }
    defer c.Close()

    w := worker.New(c, "my-task-queue", worker.Options{})
    w.RegisterWorkflow(greeting.SayHelloWorkflow)
    w.RegisterWorkflow(greeting.HelloNexusWorkflow)
    w.RegisterActivity(greeting.Greet)

    service := nexus.NewService(greeting.HelloServiceName)
    err = service.Register(greeting.HelloOperation)
    if err != nil {
        log.Fatalln("Unable to register operations", err)
    }
    w.RegisterNexusService(service)

    err = w.Run(worker.InterruptCh())
    if err != nil {
        log.Fatalln("Unable to start worker", err)
    }
}`}
```

## 4. Develop the caller Workflow

Create a file called `caller_workflow.go` that defines a Workflow which invokes the Nexus Operation.

The caller Workflow demonstrates the consumer side of Nexus.
Instead of importing handler code directly, the caller only depends on the Service contract.
This keeps the caller and handler decoupled so they can live in separate Namespaces, repositories, or even teams.

`workflow.NewNexusClient` creates a client bound to your Nexus Service and Endpoint.
`ExecuteOperation` starts the Operation and returns a future.
`fut.Get` blocks until the Operation completes and writes the result into `result`.

```go
{`package greeting

import (
    "time"

    "go.temporal.io/sdk/workflow"
)

const (
    CallerTaskQueue = "my-caller-task-queue"
    NexusEndpoint   = "my-nexus-endpoint-name"
)

func CallerWorkflow(ctx workflow.Context, name string) (string, error) {
    c := workflow.NewNexusClient(NexusEndpoint, HelloServiceName)

    fut := c.ExecuteOperation(ctx, HelloOperationName, HelloInput{Name: name}, workflow.NexusOperationOptions{
        ScheduleToCloseTimeout: 10 * time.Second,
    })

    var result string
    if err := fut.Get(ctx, &result); err != nil {
        return "", err
    }
    return result, nil
}`}
```

## 5. Create the caller Namespace and Nexus Endpoint

Before running the application, create a caller Namespace and a Nexus Endpoint to route requests from the caller to the handler.
The handler uses the `default` Namespace that was created when you started the dev server.

Namespaces provide isolation between the caller and handler sides.
The Nexus Endpoint acts as a routing layer that connects the caller Namespace to the handler's target Namespace and Task Queue.
The endpoint name must match the `NexusEndpoint` constant defined in `caller_workflow.go` from step 4.

Make sure your local Temporal dev server is running (`temporal server start-dev`).

```bash
temporal operator namespace create --namespace my-caller-namespace
```

```bash
{`temporal operator nexus endpoint create \\
  --name my-nexus-endpoint-name \\
  --target-namespace default \\
  --target-task-queue my-task-queue`}
```

## 6. Run and Verify

Create `caller/main.go` to start the caller Worker and execute the Workflow.

This brings everything together: the caller Worker hosts `CallerWorkflow`, which uses the Nexus client to invoke `say-hello` on the handler side.
The full request flows from the caller Workflow, through the Nexus Endpoint, to the handler Worker running `HelloNexusWorkflow` (which calls `SayHelloWorkflow`), and back to the caller.

**Ensure the `nexus-rpc` dependency is synchronized:**

In a terminal, from within your project:

```bash
go mod tidy
```

**Run the application:**

1. Start the handler Worker in one terminal:

```bash
go run worker/main.go
```

2. Run the caller in another terminal:

```bash
go run caller/main.go
```

You should see:

```
Workflow result: Hello Temporal
```

Open the [Temporal Web UI](http://localhost:8233) and switch between Namespaces to see both Workflow Executions.
In `my-caller-namespace`, find the `CallerWorkflow` execution — you should see `NexusOperationScheduled`, `NexusOperationStarted`, and `NexusOperationCompleted` events in its history.
In `default`, find the `HelloNexusWorkflow` execution that was started by the Nexus Operation.

```go
{`package main

import (
    "context"
    "log"

    "my-org/greeting"

    "go.temporal.io/sdk/client"
    "go.temporal.io/sdk/worker"
)

func main() {
    c, err := client.Dial(client.Options{
        Namespace: "my-caller-namespace",
    })
    if err != nil {
        log.Fatalln("Unable to create client", err)
    }
    defer c.Close()

    w := worker.New(c, greeting.CallerTaskQueue, worker.Options{})
    w.RegisterWorkflow(greeting.CallerWorkflow)

    if err := w.Start(); err != nil {
        log.Fatalln("Unable to start worker", err)
    }
    defer w.Stop()

    wr, err := c.ExecuteWorkflow(context.Background(), client.StartWorkflowOptions{
        ID:        "nexus-caller-workflow",
        TaskQueue: greeting.CallerTaskQueue,
    }, greeting.CallerWorkflow, "Temporal")
    if err != nil {
        log.Fatalln("Unable to execute workflow", err)
    }

    var result string
    if err := wr.Get(context.Background(), &result); err != nil {
        log.Fatalln("Unable to get workflow result", err)
    }
    log.Println("Workflow result:", result)
}`}
```

## Next Steps

Now that you have a working Nexus Service, here are some resources to deepen your understanding:

- **[Go Nexus Feature Guide](/develop/go/nexus/feature-guide)**: Covers synchronous and asynchronous Operations, error handling, cancellation, and cross-Namespace calls.
- **[Nexus Operations](/nexus/operations)**: The full Operation lifecycle, including retries, timeouts, and execution semantics.
- **[Nexus Services](/nexus/services)**: Designing Service contracts and registering multiple Services per Worker.
- **[Nexus Patterns](/nexus/patterns)**: Comparing the collocated and router-queue deployment patterns.
- **[Error Handling in Nexus](/nexus/error-handling)**: Handling retryable and non-retryable errors across caller and handler boundaries.
- **[Execution Debugging](/nexus/execution-debugging)**: Bi-directional linking and OpenTelemetry tracing for debugging Nexus calls.
- **[Nexus Endpoints](/nexus/endpoints)**: Managing Endpoints and understanding how they route requests.
- **[Temporal Nexus on Temporal Cloud](/cloud/nexus)**: Deploying Nexus in a production Temporal Cloud environment with built-in access controls and multi-region connectivity.